In one form of conventional variable speed pulleys, a movable pulley sheave half is urged toward a fixed sheave half by a spring. When a V-belt received in the pulley groove defined by the confronting surfaces of the sheave halves is urged radially inwardly, as by an increase in tension force applied in the belt, the belt overcomes the biasing force of the spring, permitting the movable sheave to move away from the fixed sheave and thereby reduce the effective diameter of the pulley, and thereby automatically vary the speed of the pulley driven by the belt.
It has further been known to provide means for axially moving the movable sheave half as a function of the speed of the pulley. One such structure utilizes force-applying means having a variable extension parallel to the shaft axis proportional to the centrifugal force acting thereupon as a result of the speed of rotation of the pulley. As described in Takano et al. co-pending application Ser. No. 067,691, filed June 29, 1987 now U.S. Pat. No. 4,789,578, the force-applying means comprises a plurality of small spherical elements, and a spring providing a change in the relative rotational position of the surfaces for varying the arrangement of the force-applying means as an incident of relative movement between the first and second sheaves angularly about the axis of rotation.
A problem has been noted with respect to the above described pulleys. Particularly, the belt has a tendency to fall into the belt pulley with tension generated in the belt. For example, when such a variable speed pulley is used to drive auxiliary units of an automobile, it is desirable to select the speed ratio of the driver pulley versus the driven pulley to be 1.1, or slightly higher, at engine running speeds of 2000 rpm or less. Generally, however, the torque generated at the driver pulley side is high in the range between idling speed and 2000 rpm, but at speeds higher than 2000 rpm, the torque decreases sharply. Resultingly, because of belt tension, the belt falls into the belt receiving groove of the driver side variable speed pulley which causes deceleration, resulting in shortage of speed required for the driven side variable speed pulley.
It has been proposed that such problems can be eliminated, for example, by increasing the spring thrust. However, this results in the necessity of generating larger centrifugal thrusts in response to overcome the increased spring thrust.
The present invention is intended to overcome these and other problems associated with prior variable speed pulleys.